1. Field of the Invention
The present invention relates to a projector that projects an image on a screen or another projection object and photographs the projected image. More specifically the invention pertains to a technique of accurately identifying the position of maximum brightness in the photographed image even in an inclined attitude of the projector to the projection object.
2. Description of the Related Art
Various projectors have been proposed to photograph a projected image on a screen or another projection object with, for example, a CCD (Charge Coupled Device) camera and to adjust the zoom and the focus and correct a trapezoidal distortion of the projected image (keystone correction) based on the photographed image.
One of such projectors is disclosed in Japanese Patent Laid-Open Gazette No. 2004-312690. This prior art projector analyzes a photographed image, identifies the position of maximum brightness in the photographed image, and makes keystone correction according to the identified position of maximum brightness.
In the state of elevation projection of the projector that makes keystone correction based on the identified position of maximum brightness, that is, in an inclined attitude of the projector to the projection object, the following problems arise due to the reflection of projection light from the projection object. In the following description, the luminance value is used as an index of brightness.
FIG. 5 is a perspective view showing a non-elevation projection state of a conventional projector.
A projector PJ shown in FIG. 5 projects an all-white image as an adjustment pattern image G for keystone correction on a screen Sc or a projection object and photographs the adjustment pattern image G projected on the screen Sc. The projector PJ identifies the position of maximum luminance in the photographed image and makes keystone correction.
The projector PJ is located below the screen Sc not to block the user's view. The projection optical system in the projector PJ has a lens shift to prevent a trapezoidal distortion of a projected image even in the state of projection from this lower location. The intersection between the screen Sc and the optical axis of the optical system in the projector PJ shown by the thick arrow (hereafter this intersection is called ‘optical axis point’) is deviated downward from the center of the projected adjustment pattern image G.
In the non-elevation projection state of FIG. 5, an elevation angle is equal to 0 degree. The elevation angle represents an angle in the vertical direction between the normal of the screen Sc and the optical axis of the optical system in the projector PJ.
The projector PJ has an imaging unit CA located in the vicinity of its projection optical system (not shown). The imaging unit CA includes a CCD camera and takes an image of the screen Sc including the area of the projected adjustment pattern image G and generates RGB image data of the respective pixels included in the photographed image.
The projector PJ has an automatic exposure adjustment function. A target average luminance of all pixels included in image data of a photographed image is set as a target exposure in the projector PJ. The automatic exposure adjustment function of the projector PJ calculates an average luminance of all the pixels based on the image data of the photographed image taken with the imaging unit CA and adjusts at least one of the shutter speed, the gain, and the aperture in the imaging unit CA to make the calculated average luminance approach to the target exposure or the target average luminance.
The luminance varies in a range of 0 to 255, and the target exposure set in the projector PJ is equal to ‘50’ in the state of FIG. 5.
FIGS. 6(A) and 6(B) show the non-elevation projection state of the conventional projector and a luminance distribution of an image photographed in the non-elevation projection state.
FIG. 6(A) is a side view showing the non-elevation projection state of FIG. 5. The graph of FIG. 6(B) shows a distribution of luminance values of respective pixels on a horizontal line L1 including the optical axis point of FIG. 5 in the image photographed in the non-elevation projection state of FIG. 6(A). The abscissa and the ordinate of FIG. 6(B) respectively denote the pixel position in the horizontal direction and the luminance value.
As shown in the graph of FIG. 6(B), the luminance distribution has a steep peak at the position of the optical axis point. The shorter distance between the screen Sc and the projector PJ in the coverage of the projection light of the projector PJ causes the higher luminance in the photographed image. Since the projector PJ has no inclination in the horizontal direction in the state of FIG. 6(A), the luminance distribution has a peak at the position of the optical axis point. The specular reflection of the projection light from the screen Sc goes toward the projector PJ. The luminance value thus abruptly increases at the position of the optical axis point and gives a steeper peak in the luminance distribution.
FIGS. 7(A) and 7(B) show an elevation projection state of the conventional projector and a luminance distribution of an image photographed in the elevation projection state.
FIG. 7(A) is a side view showing the elevation projection state of the projector. The graph of FIG. 7(B) shows a distribution of luminance values of respective pixels on a horizontal line including the optical axis point in the image photographed in the elevation projection state of FIG. 7(A). The abscissa and the ordinate of FIG. 7(B) are identical with those of FIG. 6(B) and are not specifically mentioned here.
As mentioned above, the projection optical system of the projector PJ has a lens shift. On some occasions, even when the projection lens is shifted to the allowable limit of the lens shift, the projector PJ may still block the user's view. In such cases, the projector PJ is located further below the screen Sc and is inclined at an elevated angle to the screen Sc as shown in FIG. 7(A). The projector PJ is located at an elevation angle of 10 degrees in the state of FIG. 7(A).
As in the state of FIG. 6, the target exposure set in the projector PJ is equal to ‘50’ in the state of FIG. 7.
As shown in the graph of FIG. 7(B), the luminance distribution has a peak at the position of the optical axis point. Unlike the luminance distribution of FIG. 6(B), however, the luminance distribution of FIG. 7(B) has a gentler peak and an indistinct peak position.
As mentioned previously, the shorter distance between the screen Sc and the projector PJ in the coverage of the projection light of the projector PJ causes the higher luminance in the photographed image. The luminance distribution of FIG. 7(B) accordingly has a peak at the position of the optical axis point as in the luminance distribution of FIG. 6(B). In the state of elevation projection, the specular reflection of the projection light, which irradiates the optical axis point, from the screen Sc goes farther from the projector PJ as shown by the thick arrow in FIG. 7(A). The peak luminance in the state of elevation projection is accordingly lower than the peak luminance in the state of non-elevation projection. This causes smaller differences in luminance between the maximum luminance position and peripheral lower luminance positions and gives a gentler peak in the luminance distribution.
The larger elevation angle causes the specular reflection of the projection light irradiating the optical axis point to go father from the projector PJ. The luminance distribution accordingly has a gentler peak and a more indistinct peak position.
In the state of elevation projection, the conventional technique may give only an indistinct peak position or maximum luminance position and fail to accurately identify the peak position in the photographed image. The inaccurate identification of the peak position may result in inadequate and inaccurate keystone correction.